1. Field of the Invention
The invention pertains to apparatus and methods for detecting neutrons and more particularly to neutron detectors containing generally parallel or concentric detecting elements made of thin sheet-like material coated with neutron-reactive materials.
2. Description of Related Art
Conventional Neutron Detector Tubes
Tubular helium-3 proportional counters have been the most common type of neutron detector in use for some years, such as described by Bonner in U.S. Pat. No. 3,102,198 and Morgan in U.S. Pat. No. 3,240,971. Boron trifluoride (BF3) gas was originally commonly used for neutron detection and was replaced by He-3 based detectors in the 1960's when He-3 became available (see, for example, Goodings and Walgate Leake in U.S. Pat. No. 3,702,409 and Bayard in U.S. Pat. No. 2,879,423). BF3 detectors present major drawbacks compared to He-3 detectors, including the toxicity of the gas, and a commonly recognized limitation of the pressure for effective operation of the detector, which limits the potential sensitivity of these detectors, compared to conventional counters that can be filled with 10 or 20 atmospheres of He-3. An alternative to using a neutron-reactive gas is to line a gas-filled chamber with a neutron-reactive solid, such as boron-10 (see, for example, Curtis et al. in U.S. Pat. No. 2,845,560 and Gleason in U.S. Pat. No. 3,956,654) or lithium-6.
Neutron detectors based on boron-10 lined tubes have attracted increased attention in recent years because of a supply shortage of He-3 gas and a consequent surge in price, making He-3 uneconomic for many neutron detection applications. A disadvantage of boron-lined tubes compared to detector tubes containing helium-3 or boron trifluoride is that the former normally has a substantially lower neutron detection sensitivity than the latter due to the self-shielding of boron-10 neutron capture reaction products by the solid coating containing the boron (other solids also experience this effect). One approach to achieving higher neutron detection efficiency with boron-lined tubes is to use a multiplicity of smaller diameter boron-lined tubes in place of a single larger tube; this approach works by increasing the solid boron surface area (and thus the neutron detection sensitivity) per unit volume of detector. Thermal neutron detection efficiencies for commercially available boron lined tubes are typically in the range of 3 to 6%, which is very low compared to the 60 to 70% thermal neutron detection efficiency of a typical two inch diameter tube detector filled with 3 atmospheres of He-3. Centronic, GE, Proportional Technologies Inc., and other companies have sought to provide a He-3 replacement using this approach, with a number of smaller diameter boron-lined tubes bundled into a single detector enclosure whose size is appropriate for substituting it in place of a He-3 detector tube or system. Drawbacks to using smaller diameter tubes include greater difficulty in coating the inside surface of the tubes with boron or other neutron-reactive material and in inserting a thin anode wire into the tube, particularly if the tube is small (e.g. Proportional Technologies' straw tubes are 4 mm in diameter and can be up to six feet long). Although the companies mentioned have developed engineering solutions for manufacturing these detectors, it remains the case that the smaller the tube diameter, the greater the number of tubes required to provide a performance-equivalent replacement for a He-3 tube detector of a given size, resulting in an obvious cost increase.
Boron Straw Tubes
Lacy describes a boron coated straw neutron detector in U.S. Pat. No. 7,002,159. Conceptually, the boron coated straw neutron detector is a type of tubular boron-lined proportional counter with some advantages over traditional boron-lined tubes, one of which is the use of a thin walled straw that can be fabricated by rolling up a thin boron-coated sheet instead of using a metal tube to form the body of the detector. One advantage of this process is that it allows the inner surface of the tube to be coated with boron while in a substantially flat form before being rolled into a small diameter tube. On the other hand, boron coated straw neutron detectors normally use a central anode wire just as boron-lined neutron counters do, and this approach does not simplify the difficult task of sliding the wire into the straw.
Boron Lined Tubes with Baffles
Dighe et al. describe two approaches to increasing neutron detection sensitivity by increasing the boron-coated surface area inside a single tube [P. M. Dighe et al, “Boron-lined proportional counters with improved neutron sensitivity,” Nuclear Instruments and Methods A 496, pp. 154-161 (2003)]. The first approach is to simply use smaller diameter boron-lined tubes, with a group of such tubes held inside a containment structure (e.g. a single larger tube), with the smaller tubes sharing a common connector for supplying the operating voltage and providing the neutron detection signal out. The second approach consists of mounting boron-coated baffles inside a tube and spaced along the length of the tube. In a later work, Dighe describes the results obtained from adding boron-lined baffles to the interior of a boron-lined tube; Dighe reports a measured neutron sensitivity almost 2.8 times higher than that of a comparable boron-lined tube containing no boron-lined baffles. [P. M. Dighe, “New cathode design boron lined proportional counters for neutron area monitoring application,” Nuclear Instruments and Methods A 575, pp. 461-465 (2007)]. Dighe et al. present pulse height spectra that show a logarithmic increase in counts with decreasing pulse amplitude within the low pulse amplitude range, making the actual neutron detection efficiency significantly sensitive to the lower level discriminator (LLD) value used to discriminate out electronic noise and gamma induced pulses from the neutron signal. Dighe does not propose any means of improving the pulse height spectrum to enhance the ability to reject electronic noise and gamma rays without losing neutron pulses.
Micro Neutron Detectors
McGregor et al. in U.S. Patent Application 2006/0043308 disclose micro neutron detectors based on components (usually two substrates with cavities in them) that are fitted together in a gaseous environment to form a gas-filled pocket that acts as a neutron detector. Neutron sensitivity is achieved by having a neutron-reactive material present in the detector, such as a thin layer of a neutron-reactive solid coating one of the interior surfaces of the detector. McGregor et al. disclose additional variations and uses of this technology in U.S. Patent Applications 2006/0023828 and 2006/0056573. In the disclosed detector configuration, McGregor et al. indicate how two generally flat surfaces serve as electrodes (namely the anode and cathode), but do not teach how one would reduce the capacitance induced noise that results when placing large panels in close proximity to each other when fabricating a large area detector, nor do they provide a means for amplifying the signal relative to this increased noise to improve the signal-to-noise ratio.
Parallel Plate Avalanche Chamber (PPAC)
A Parallel Plate Avalanche Chamber (PPAC) comprises two parallel plates that function as an anode and a cathode with ionizations created in the detector gas by a radiation particle producing an electron avalanche that is the measured signal. Thanks to the small distance between the electrodes, the entire region between the electrodes normally functions as an electron multiplication region.
Proportional counters (e.g. He-3 tube detectors) typically have a large drift region (where the electric field strength is comparatively lower) and a smaller amplification region (where the electric field strength is stronger) in which gas gain (electron multiplication) occurs. When a radiation particle interacts with the detector and deposits energy in the detector gas, it is usually the case that most or all of this energy is deposited in the drift region in the form of ionizations (ion-electron pairs). When the electrons are drawn into the amplification region by the influence of the electric field, they undergo multiplication in the amplification region. As a result, the amplified signal is essentially proportional to the number of electrons initially liberated by the radiation particle in the drift region, which in turn is proportional to the energy deposited in the detector by the radiation particle.
PPACs stand in contrast to proportional counters in this matter. In a PPAC, the entire gas chamber in the detector acts as an amplification region. A consequence of this is that an electron liberated by a radiation particle close to the cathode will experience a greater amount of gain due to having a greater distance to travel to the anode than an electron liberated closer to the anode. As a result, proportionality is lost between energy deposition by a radiation particle and the amplitude of the resulting pulse measured at the anode. This loss of proportionality is undesirable because it undermines the ability to differentiate between different types of radiation based on the amplitude of the detection pulses (e.g. neutrons are often distinguished from gammas by counting pulses above a threshold amplitude level as neutrons and rejecting pulses below that level on the presumption that they are gammas).
As PPACs use plates covering the entire side of a detector as electrodes (as opposed to electrodes having a smaller area, such as wires or mesh), their capacitance per unit area of detector size is quite high. Laboratory experiments are a common application for PPACs as their flat shape tends to make them sensitive to vibration. It only takes a small deflection of the plates comprising the detector to create a large transitory fluctuation in the capacitance, thereby producing a charge pulse spike at the input of the readout electronics. In these circumstances even a small acoustic pressure wave (sound) may generate appreciable deflection of these plates [A. Breskin et al., “A fast, bidimensional position-sensitive detection system for heavy ions,” Nuclear Instruments and Methods 148, pp. 275-281 (1978) and M. Nakhostin et al., “A fast response and γ-insensitive neutron detector based on parallel-plate avalanche counter,” Radiation Protection Dosimetry 129, pp. 426-430 (2008)].
Micromegas Detector
The Micromegas (MicroMEsh GAseous Structure) is quite similar in design to a PPAC, with the difference that a drift electrode is located a short distance above the anode, dividing the space between the anode and cathode into two regions; a drift zone (operating in ionization mode) and a signal amplification zone (operating in proportional mode). This division results in retention of proportionality between the energy deposition in the detector gas by the radiation particle interaction and the signal amplitude (for radiation particles depositing their energy in the drift zone, rather than the amplification zone, which is most of them). To enable neutron detection, a thin coating of neutron-reactive material such as boron-10 or lithium-6 can be placed on the cathode [S. Andriamonje et al., “Experimental studies of a Micromegas neutron detector,” Nuclear Instrument and Methods A 481, pp. 120-129 (2002) and S. Andriamonje et al., “New neutron detectors based on Micromegas technology,” Nuclear Instruments and Methods A 525, pp. 74-78 (2004)]. This detector design requires very precise positioning between the Frisch grid and the anode as well as significant structural rigidity of the structure to ensure stable operation of the detector. A rigid substrate such as a quartz plate is often used and standoffs are employed to hold the Frisch grid at a constant distance from the anode. A major advantage of the Frisch grid rather than an anode wire as in standard tubular proportional tubular counters (He-3 tubes) is that the electric field in the drift region can be controlled generally independently of the gain obtained in the electron multiplication zone. This also allows detection pulses to be generated from electron and ion drift between the Frisch grid and the anode (rather than between the cathode and the anode), enabling high time resolution (e.g. tens of nanoseconds or less) of use in applications such as in neutron spallation experiments. Using the standard wire approach (rather than a Frisch grid approach), the voltage potential between the anode and cathode controls both the gas gain and the electron drift velocity at the same time. The major drawback of the Micromegas is that each anode can only read from one neutron reactive surface. If one were to place an anode on each side of a single substrate, the detector capacitance would increase to the same extent as if the two anodes were separate, thus increasingly the electronic noise level, and the minimum required thickness of the substrate would increase the overall thickness of the device.
Multiwire Proportional Counters (MWPCs)
Multiwire Proportional Counters (MWPCs) are two-dimensional radiation imaging detectors that use a series of thin wires as anodes. The use of wires for anodes causes the electric field to be concentrated around the wires, creating an electron amplification region in the immediate vicinity of the wires and causing the detector to operate in proportional mode. By measuring and comparing the signal amplitude generated on the different anode wires by a single radiation particle interaction event, the position of the interaction event can be deduced. MWPCs are used primarily in laboratory settings, such as position-sensitive neutron detection in neutron scattering experiments. A drawback to traditional MWPC design is its flatness and use of long pieces of unsupported anode wires, rendering it susceptible to microphonics noise (e.g. when used in a non-laboratory environment) [G. Charpak et al., “Multiwire proportional chambers and drift chambers,” Nuclear Instruments and Methods 162, pp. 405-428 (1979), G. Giorginis et al., “A three dimensional He-recoil MWPC for fast polarized neutrons,” Nuclear Instruments and Methods in Physics Research A 251, pp. 89-94 (1986), and R. B. Knott et al., “A large 2D PSD for thermal neutron detection,” Nuclear Instruments and Methods A 392, pp. 62-67 (1997)]. Melchart et al. disclose a variant on the MWPC concept in which an MWPC was placed beside a set of mesh electrodes with an electric field between them sufficient to produce electron multiplication. A layer of gadolinium (a neutron-reactive material) was included in the detector on one side of the gas chamber to provide neutron sensitivity. [G. Melchart et al., “The multistep avalanche chamber as a detector for thermal neutrons,” Nuclear Instruments and Methods 186, pp. 613-620 (1981)].
More recently, Wang and Morris [C. L. Morris et al., “Multi-wire proportional chamber for ultra-cold neutron detection,” Nuclear Instruments and Methods in Physics Research A 599, pp. 248-250 (2009) and Z. Wang et al. “Multi-layer boron thin film detectors for neutrons”, in press in Nuclear Instruments and Methods in Physics research A] adapted a planar He-3 based multi-wire detector for use with boron coated cathode as neutron reactive conversion media in a multi-layer configuration. The authors found that obtaining sufficient robustness against electronics noise and microphonics for successful operation of the detector device required operation with gas gain, which required very high dimensional tolerance in positioning the anode and cathode planes. The authors indicate that they achieved a 100 μm precision in wire spacing and plane spacing in the He-3 based detector with a 2.5 mm spacing between the anode plane and the cathode planes. The boron-based device had about 10% or less variation in spacing between the different layers with the separation between the anode and the cathodes being 3.3 mm thus this corresponds to a precision of 0.33 mm. The authors present a pulse height spectrum for a single layer showing (as expected) sharp peaks corresponding to the two types of reaction products released by the neutron capture reaction in boron-10. However, in the multi-layer device (combining measurements from four boron layers), the spectrum lost the sharp peaks indicating the gain may be different for each layer due to variations in anode-cathode spacing and thus this design may have issues with repeatability. These tolerance issues briefly introduced by the authors will have increasingly greater impact on devices in which the distance between the cathodes and the anodes is smaller.